A unidirectional error of 1/100th of a second would accumulate around a
minute and a half per day. It's been a long time since I laid eyes on a
mechanical pendulum clock. I remember the clock in my childhood home kept
better time than that. ( I became odd very early. I compulsively compared
the clock time to WWV time at least once a day. I had been using the time
service from the phone company. I felt defrauded when I discovered (via WWV)
that the time from the local telco's dialup time service was just a rough
(very rough) approximation of the time.
Dale
-----Original Message-----
From: paul swed
Sent: Sunday, December 01, 2013 12:14 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Using a UBlox NEO-6 GPS module for calibrating a
PIC microprocessor based timer.
Luke
I would believe that any of the GPS receivers would be great for what you
are doing. (Most likely just opened a bag of worms.) Indeed they have
jitter and all the things mentioned. But if the discussion is about a
mechanical clock then its accuracy is many decades lower than any of this
discussion. Then you have the temperature coefficients s of the clock
mechanism that limits your overall accuracy.
In you post you are measuring nano-seconds. Seems far far to fine grain as
I picture a pendulum clock in my mind. I would speculate 1/100-1000 of a
second is reasonable. Because the pendulum has mass and only varies over
significant time the funnies with gps should generally go away.
By the way purchase a RB reference on ebay. Fire it up and set it in the
bottom of the clock. Now you have a RB clock. ;-)
Anyhow now I am in trouble.
Regards
Paul
WB8TSL
On Sun, Dec 1, 2013 at 9:46 AM, Mike Garvey <[email protected]> wrote:
Luke,
Seems that you might indeed become a time nut, though most of those on the
list are focused on atomic clock topics and the time precision that atomic
clocks provide.
That said, sounds like you have a good start. While I have not used the
GPS
module you mention, GPS timing sources typically show timing accuracies of
20 to 100 ns. The short term noise arises from the signal-to-noise of the
"as received" signals, so the 99% accuracy of 60 ns likely means that you
can assume that all of the pulses are within +/-100 ns. The time noise is
"white" for GPS signals. Note also that this is time accuracy, not
frequency accuracy. Frequency accuracy is maintained by GPS Ground
Control
Segment with reference to the US Naval Observatory Time Scale.
At the risk of unraveling your existing measurement architecture, you
might
consider measuring the clock phase (pendulum passing) with respect to the
GPS pulse. This would be a time interval measurement with an accurate
(GPS)
reference pulse and a time interval based upon your TCXO. This reduces
the
burden of calibrating the TCXO and will eliminate concerns about the
frequency accuracy and stability of the TCXO. For example, if your TCXO
is
off frequency by 1 ppm, you would get a non-cumulative error of 1 ppm in
the
time interval measurement rather than a 1 ppm error in the clock rate.
The
1 ppm TCXO error drops out of the solution for rate of the clock; only
changes in the TCXO frequency enter and they are typically much smaller.
Hope this is useful; others on the list will likely have other input.
Mike
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf Of Luke Mester
Sent: Sunday, December 01, 2013 2:54 AM
To: [email protected]
Subject: [time-nuts] Using a UBlox NEO-6 GPS module for calibrating a PIC
microprocessor based timer.
One of my hobbies is collecting and repairing mechanical clocks. I was
looking at buying one of the specialized electronic timers used to measure
the performance of mechanical clocks. I really couldn't justify the cost
just for hobby use.
Since I have electronics and programming skills I decided to build my own
timer using a PIC chip. This became a much bigger project than I expected!
I have my clock timer running and have most of the software features that
I
need working. I then realized that I need some way to calibrate it and
verify it's accuracy.
I didn't have any source of accurate time available. After searching the
internet and finding this mailing list I decided to try a GPS module. I
bought a $20 module from DX.com. It has a built in antenna, voltage
regulator, serial interface and most important, a 1 PPS output.The GPS is
a
UBlox NEO-6M. After reading the specs on this module I see that they claim
a
99% accuracy of <60ns for the time pulse signal. What does this mean?
What about the other 1%? How much variation can the time pulse have? If
it's
really 60ns it's much better than I need.
I'm hoping that some of the time experts on this mailing list can give me
some idea what to expect from this GPS module. Also, if there are any
settings that I should change to get better timing performance. There are
a
huge number of settings available when I run it's configuration program. I
have no idea what most of them do.
I'm using one of the hardware timers on the PIC chip to measure the time
interval. The PIC is running with a 100ns (10MHz) instruction cycle. The
timer will provide 100ns resolution. I'm getting occasional variations of
about a microsecond. Because I'm using interrupt driven code to capture
the
timer value there will be some unavoidable jitter in the timing. I was
expecting about 4 or 5 cycles (400ns - 500ns) but I'm getting more than
twice that. Is it safe to assume that these are due to problems in my
hardware or software? Is this from variations in the GPS PPS output? Maybe
I'm just not interpreting the data correctly.
Below are links to some data plots:
Four plots are shown. The first two are the Rate and Beat error that my
timer reports while monitoring the GPS PPS signal. Rate is normally the
average of two beats ( two time interval samples). If a clock is not in
beat
(the tick and tock take different amounts of time) the displayed rate
would
jump back and forth. Averaging two beats eliminates this jump. I have
disabled this average in my code so that the rate is now showing each beat
and not the average of two. I turned it off because I expect that this
averaging could hide possible problems with my timer. Beat error is the
difference between the two beats. This shows the rate change for each pair
of beats. This is needed so that you can get the clock pendulum or balance
wheel adjusted properly.
Raw Data <http://mesterhome.com/clock/data/RawData.png>
Average Data <http://mesterhome.com/clock/data/AveData.png>
Average data has been filtered with a 100 sample running average. The plot
looks really good. The average is just hiding the instability.
I also noticed variations that appeared to be due to temperature changes.
I
borrowed a temperature data logger from work and did some testing. The
temperature and rate graphs track perfectly. I can see my furnace cycling
and my programmable thermostat moving the temperature setting up and down!
That got me interested in trying a TCXO instead of the standard crystal
that
I was using. A $3.00 TCXO from EBay made a huge difference! Both of these
plots have the running average applied. You can't see the temperature
changes with the raw data.
Crystal <http://mesterhome.com/clock/data/RTvTP.png>
TCXO <http://mesterhome.com/clock/data/RTvTPTC.png>
In case anyone is interested, here is a link to a data file captured from
the clock timer. It's in CSV format. The first column labeled "Rate" is
the
time for each beat of the clock. "Rate Avg" is a running average rate and
"Beat E" the beat error.
Data file <http://mesterhome.com/clock/data/tcxo.csv>
Finally, I think I might be turning into a time nut! For the clocks that I
work with this timer is already far better than I need. Millisecond
accuracy
Is good enough to test most mechanical clocks. Microsecond is great! I
know
that a microprocessor based timer is capable of better performance. I then
had the problem of what I could use to measure the performance of my
timer.
I Needed a better clock than my timer. Now I'm wondering if this cheap
Chinese GPS is good enough. I'm having fun tweaking the hardware and
software to see just how good I can get it to perform!
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